"You can see so many ways that it could fail and it works and you're like, 'Wow, OK, it didn't fail'," Musk said, laughing, from his company's headquarters in Hawthorne, California.

"I think anyone who's been involved in the design of a really complicated machine can sympathise with what I'm saying."

The goal for SpaceX, he told reporters, will be to repeat the success on future flights.

The unmanned supply ship scored a bull's-eye with its arrival, splashing down into the ocean about 800km off Mexico's Baja California peninsula. A fleet of recovery ships quickly moved in to pull the capsule aboard a barge for towing to Los Angeles.

It was the first time since the shuttles stopped flying last summer that NASA got back a big load from the space station, in this case more than half a ton of experiments and equipment.

Thursday's dramatic arrival of the world's first commercial cargo carrier capped a nine-day test flight that was virtually flawless, beginning with the May 22 launch aboard the SpaceX company's Falcon 9 rocket from Cape Canaveral and continuing through the space station docking three days later and the departure a scant six hours before hitting the water.

Companies compete

Dragon resembled NASA's Mercury, Gemini and Apollo spacecraft of the 1960s and 1970s as its three red-and-white striped parachutes opened. Yet, it represents the future for American space travel now that the shuttles are gone.

"This successful splashdown and the many other achievements of this mission herald a new era in US commercial spaceflight,'' NASA Administrator Charles Bolden said in a statement.

Alan Lindenmoyer, manager of NASA's commercial crew and cargo programme, was emotional as he turned to Musk and assured him that NASA was now his customer and that resupply services were about to unfold on a regular basis.

President Barack Obama has led the charge to commercial spaceflight. He has asked for routine orbital flights turned over to private business so the space agency can work on getting astronauts to asteroids and Mars. Towards that effort, NASA has provided hundreds of millions of dollars in seed money to vying companies.

NASA astronauts are now forced to hitch rides on Russian rockets from Kazakhstan, an expensive and embarrassing outsourcing, especially after a half-century of manned launches from US soil.

It will be up to SpaceX or another US enterprise to pick up the reins. Several companies are jockeying for first place.